1. Field of the Invention
The present invention relates to a spectrum spread communication system.
2. Description of the Related Art
FIG. 12 is a block diagram showing an example of a conventional spectrum spread communication system.
The spectrum spread communication system shown in FIG. 12 using a radio LAN constructed to the mode specified by IEEE 802.11.
This prior art shown in FIG. 12 comprises a transmitter 210 for transmitting a predetermined signal and a receiver 220 for receiving the signal transmitted from the transmitter 210. The transmitter 210 is provided with a terminal 215 for receiving an input of serial data, an S/P converter 211 for converting the serial data input from the terminal 215 into parallel data, a code generator 213 for generating a Barker code as a spread code multiplied by the data signal which has been converted by the S/P converter 211, a spread device 212 for multiplying the Barker code by the data signal converted by the S/P converter 211, an analog transmission process device 214 for modulating the data signal multiplied by the Barker code in the spread device 212 and a terminal 216 for transmitting the data signal modulated by the analog transmission process device 214 to the receiver 220. The receiver 220 is provided with a terminal 221 for receiving a data signal transmitted from the transmitter 210, an analog reception process device 225 for amplifying the data signal received by the terminal 221 and converting the data signal into a base band signal, a correlator 226 for taking a correlation between the Barker code multiplied by the data signal converted into the base band signal in the analog reception process device 225 and the Barker code at a timing of the code generator 213 and outputting a data signal based on a resultant correlation, a demodulator 227 for demodulating the data signal output from the correlator 226, a P/S converter 228 for converting the data signal (parallel data) demodulated by the demodulator 227 into serial data and a terminal 229 for outputting the serial data which have been converted through the P/S converter 228.
FIG. 13 is a block diagram showing a construction example of the correlator 226 as shown in FIG. 12.
The prior art correlator shown in FIG. 13 comprises a plurality of delay devices 401 each at a delayed chip rate, multipliers 402 for +/-1 multiplication and adders 403 for addition. Multiplier factors +1 and -1 of the multipliers 402 are identical to the Barker code sequence.
A communication operation of the above-constructed spectrum spread communication system is hereinafter described. It is assumed that a signal transmission rate is at 2 Mbps and an 11-bit Barker code as a spread code is used for the spectrum spread.
When the serial data are input to the terminal 215, the input serial data are converted into parallel data by the S/P converter 211.
A Barker code as a spread code is generated by the code generator 213 which is multiplied by the data signal converted by the S/P converter 211 and then output at a chip rate of 11 MHz.
Then in the spread device 212, the Barker code is multiplied by the data signal which has been converted by the S/P converter 211.
In the analog transmission process device 214, the data signal which has been multiplied by the Barker code in the spread device 212 is modulated for transmission to the receiver 220.
The data signal modulated by the analog transmission process device 214 is transmitted to the receiver 220 via the terminal 216.
The data signal transmitted from the transmitter 210 is received by the receiver 220 via the terminal 221.
The data signal received via the terminal 221 is amplified and converted into a base band signal in the analog reception process device 225.
Then in the correlator 226, a correlation is between the Barker code multiplied by the data signal which has been converted into the base band signal by the analog reception process device 225, and the Barker code at a timing of the code generator 213. Based on the obtained correlation result, the data signal is output.
Output of the correlation result provided by the correlator 226 is hereinafter described.
FIG. 14 shows an output of the autocorrelation value of the 11-bit Barker code.
FIG. 15 shows an output signal of the correlator 226 when delay distortion exists.
When the timing is at 0, the autocorrelation becomes +1. When the timing is not 0, the autocorrelation becomes -1/11 value or zero. A delay wave is generated on the transmission path owing to multipath distortion. As FIG. 15 shows, the delay wave is divided by each chip at every spectrum spread through the spread device 212. The chip signal having the highest correlation value is only extracted in the correlator 226 and the chip signal having low correlation values (i.e., chip signal resulted from the delay wave) is removed. As a result, the distortion resulted from the delay wave is reduced.
After the correlator 226 outputs the data signal based on the correlation result, the demodulator 227 demodulates the data signal. Then the demodulated data signal is converted into serial data by the P/S converter 228.
Finally the converted serial data are output from the terminal 229.
In order to increase the transmission rate in the conventional spectrum spread communication system, the symbol rate is required to be increased or the number of bits added per symbol is required to be increased. These processes for increasing the transmission rate have the following problems.
(1) Increasing the symbol rate: PA0 (2) Increasing added number of bits per symbol:
The spread ratio is specified to be set to 10 times or more in an ISM band at 2.4 GHz in Japan. Additionally the band width is limited to 26 MHz. So the symbol rate cannot be doubled or it will exceed the conventional rate.
The added number of bits per symbol can be increased through a demodulation process such as multiphase PSK and QAM instead of QPSK. Accompanied with the increase in the number of bits, a bit error rate is increased under the influence of the delay distortion. The operation process is, thus, required to have higher accuracy for reducing such error. However it is practically and technologically difficult to realize the high accuracy of the operation.
For example, multiplexing the signal to 16-value or more QAM is required to obtain the transmission rate 2 times higher than QPSK. The 16-value QAM requires peak power 10 times more than that required by the QPSK. Also the bit error rate becomes large to the amplitude distortion, thus exerting substantially a heavy load to the device when trying to obtain the same accuracy as that of the QPSK. It becomes susceptible to the distortion on the transmission path, resulting in reduced service area.